Hi, I have a master-detail relationsip between a site and a venue field.  With my current code below the page preloads the picklists with these values.


How to I get the ID of the selected site object so that I can populate the venues related to that particular site? 

Markup*
                    <apex:selectList required="true" label="Site" value="{!selectedSite}" multiselect="false" size="1" >
                        <apex:selectOptions value="{!SitesOptions}"></apex:selectOptions>
                    </apex:selectList>
                    
                    <apex:selectList required="true" label="Venue" value="{!selectedVenue}" multiselect="false" size="1" id="myVenues">
                        <apex:selectOptions value="{!VenueOptions}" ></apex:selectOptions>
                    </apex:selectList>

ControllerCode*
    public User currentUser = [select id, name, business_unit__c from user where id =: userInfo.getUserId()];
    public String currentUserBusinessUnit = currentUser.Business_Unit__c;
    public String selectedSite{get;set;}
    public String selectedVenue{get;set;}
    
    public List<selectOption> getSitesOptions(){
        List<selectOption> siteOptions = new List<selectOption>();
        List<Site__c> siteList = new List<Site__c>();
        siteList = [SELECT Id,Name FROM Site__c WHERE business_unit__c =: currentUserBusinessUnit];
        
        for(Site__c s : siteList){
            siteOptions.add(new selectOption(s.Id,s.Name) );
        }
        return siteOptions;
    }
    
    public List<selectOption> getVenueOptions(){
        List<selectOption> venueOptions = new List<selectOption>();
        List<Venue__c> venueList = new List<Venue__c>();
        venueList = [SELECT Id,Name FROM Venue__c];
        
        for(Venue__c v : venueList){
            venueOptions.add(new selectOption(v.Id,v.Name) );
        }
        return venueOptions;
    }